Conventionally, an embedded device that performs map drawing, such as a car navigation system, is typically configured so as to integrally include a storage having map data used for drawing therein, a drawing device to draw a map, and a display device to display the map. On the other hand, because there are a case in which the number of display devices in a car navigation system is increased to two or more and a case in which a map is displayed also on an information device carried in by a passenger (e.g., a tablet computer or the like), there is an increasing demand to separate a display device from a storage having map data therein.
In recent years, since improvements in the processing capabilities of embedded devices have been provided and it has become popular for embedded devices to have a network connection function, the storage, the drawing device, and the display device can be disposed separately as pieces of independent equipment, respectively. However, when a map image itself is transmitted to the display device via a network, the amount of data transmitted increases. In a car in which a car navigation system is mounted, because it is typical to use a network used for transmission of a map image also for transmission of other data, a reduction of the influence upon other communications by reducing the amount of data transmitted as much as possible is desired.
Further, in the case of separately disposing the storage, the drawing device, and the display device as pieces of independent equipment, respectively, there is a case in which information about the resolution which is originally consistent among the devices becomes inconsistent. For example, there can be a case in which only the display device is replaced by a one having a higher or lower resolution. Even in such a case, displaying an image in such a way that the image is legible to the user of the car navigation system is desired.
As a system that displays such a map, patent reference 1 discloses a technique of dividing data required for map drawing into part data required for display and drawing data including screen component data, and also reducing the amount of data transmitted by not retransmitting part data which are already transmitted.
Further, patent reference 2 discloses a vehicle-mounted information system that can display, on each of display devices having different image display capabilities, an image having an amount of information which is suitable for the image display capability by performing a process of making the image contents to be displayed on the display device have the amount of information suitable for the image display capability on the basis of the information about the image display capability.